The mid-IR supercontinuum generation has attracted much attention during the recent years because many unique molecular absorption bands of most of the molecules exist in this domain. Additionally, mid-IR supercontinuum light sources are expected to have potential applications including astro-photonics, bio-photonic diagnostics, nonlinear spectroscopy, infrared imaging and sensing. For high spatial resolution imaging a spatially coherent supercontinuum light source is desirable. The soft-glass optical fiber is the promising medium for the design and development of a high spatially coherent mid-IR light source with the high brightness. Earlier, the broadband mid-IR supercontinuum generation has been reported using the optical fibers in different materials including tellurite, and chalcogenide, but, its coherence property has not been demonstrated extensively. In this work, we experimentally demonstrate the mid-IR supercontinuum spectrum spanning ⁓1.6 μm to 3.7 μm using a 3 cm long tapered chalcogenide step-index optical fiber pumped with femtosecond laser pulses at 2.6 μm. To justify the experimentally obtained results, a numerical simulation also carried out for the same fiber and pulse parameters. The measured supercontinuum spectrum matches well with the simulated spectrum and generated supercontinuum spectrum is highly coherent within the whole spectral range of the supercontinuum generation.
Supercontinuum (SC) has been applied in many applications such as optical coherence tomography, high-precision spectroscopy and frequency metrology [1]. SC can be generated in highly nonlinear fibers by launching high intensity laser pulses into these fibers. The dynamics of SC generation (SCG) closely relates to the chromatic dispersion of the fibers [2]. In the normal dispersion regime, the spectrum broadening dynamics is mainly based on self phase modulation and optical wave breaking which are self-seeded processes. Thus, the output SC preserves its high coherence. Highly coherent and broadband mid-infrared SCG in the all-normal dispersion regime was demonstrated by pumping at 8, 10, and 12 μm [3]. However, only few laser sources are able to provide such long wavelengths. Moving the pumping laser wavelengths to around 1.5 or 2 μm will be more attractive because many commercial fiber lasers are available. In this report, we propose a novel tellurite fiber for supercontinuum generation with a pumping laser at 2 μm. The fiber is obtained by adding six solid rods around the core of a step-index fiber. Such fiber is called all-solid hybrid microstructured optical fiber (ASHMOF). The fiber possesses flattened chromatic dispersion from 2 to 4 μm. Successful fabrication of the ASHMOF was done with an in-house drawing tower. Using a laser pumping at 2 μm into the ASHMOF, highly coherent and high spectral flatness supercontinuum spanning a range from 1.4 to 3.0 μm at - 20 dB level was experimentally generated. Such broad and highly coherent SC will be valuable for applications as optical coherence tomography, ultrafast transient absorption spectroscopy, etc.
High peak power mode locked fiber lasers are effective tools for many applications like optical metrology, biomedical imaging, micromachining and so on. All-fiber architecture and high pulse energy mode locked fiber lasers are very attractive to these applications due to their compactness and robustness. In recent years, an attractive mode-locking scheme based on Mamyshev regenerator was demonstrated to realize high pulse energy mode locked pulse output leading to breakthroughs in the mode locked fiber laser performance. We experimentally demonstrate an all-fiber linear Mamyshev regenerator operating at 1550 nm. Self-phase modulation and off-set spectral filtering provide high peak power pulse pick-up effect in the laser cavity and impel the laser to operate in the mode locking regime. With properly setting of the parameters, the all-fiber Mamyshev regenerator can achieve self-starting of the mode locking easily. No external pulse seeds or auxiliary starting arms are needed for the self-starting of the mode locking, which makes the laser very convenient to operate. Pulses with maximum energy of ~18 nJ and pulse width of 230 fs were achieved. The pulse width almost keeps unchanged with increasing in the pump power and the output power increases almost linearly with the pump power. The spectra from the two outputs with different pump powers were experimentally investigated as well. This high pulse energy Mamyshev regenerator can be used as a high quality and cost-effective laser source for many applications.
A linear mode-locked fiber laser based on the semiconductor saturable absorption mirror was demonstrated to realize mode locking in group velocity locked vector soliton regime and polarization rotation vector soliton regime. The highly fixed birefringence was introduced into the linear fiber laser cavity by using a piece of polarization maintaining Erdoped fiber. The vector nature of pulse of the mode-locked fiber laser was experimentally studied. It turned out that, this birefringence enhanced mode-locked fiber laser can generate both group velocity locked vector soliton and polarization rotation vector soliton though slightly changing the birefringence of the laser cavity. The experimental results reveal the fundamental physics of group-velocity locked vector soliton and polarization rotation vector soliton generation and demonstrate that the linear birefringence enhanced fiber laser is a good platform for investigating the vector soliton.
The mid-infrared (mid-IR) spectral region is very important topic of research because the molecular fingerprint of most of the molecules find in this region. Therefore, the mid-IR supercontinuum has been of great interest for the application of spectroscopic chemical sensing, metrology, and hyper-spectral imaging. Presently available mid-IR light sources such as optical parametric oscillators, quantum cascade lasers, thermal emitters and synchrotron radiation are not suitable for such mid-IR applications where we require broadband, spatial coherence, portable and high brightness of laser sources. Supercontinuum generation using the optical fibers has been one of the prominent approaches to obtain broadband mid- IR light sources. In this work, we have numerically investigated an all-normal dispersion engineered tapered tellurite step-index fiber structure for the generation of coherent supercontinuum spectrum in the mid-IR region. Supercontinuum spectrum spanning 1.04 – 4.34 μm is obtained by 200 fs laser pulse pumping of coupled peak power of 44 kW at 2.0 μm. Broadband and coherent mid-IR supercontinuum light is generated in a 4 cm long tapered step-index tellurite fiber. Coherent mid-IR supercontinuum spectrum reported in this work is expected to have potential applications for a variety of important applications in various fields including imaging, early cancer detection, sensing, and precision spectroscopy.
A W-type co-axial chalcogenide optical fiber structure is designed and numerically analysed for the broadband and highly coherent supercontinuum sources in the mid-IR region. The structural parameters of the designed W-type optical fiber are optimized to obtain small absolute group velocity dispersion in broad spectral range in the mid-IR region. The proposed W-type fiber structure possesses a flat dispersion profile with the flatness of the dispersion of ±2.45 ps/nm/km in the spectral range of 4.9 – 12.6 μm. The broadband and coherent mid-IR supercontinuum spectrum extending from 2.28 μm to 15.52 μm at -40 dB level is obtained using a 4 cm long chalcogenide W-type fiber pumped by 200 fs laser pulse of peak power of 10 kW at 7 μm. The average coherence property of the supercontinuum spectrum is almost unity in the full spectral range for the chalcogenide W-type fiber. Such broad and highly coherent mid-IR supercontinuum spectrum is very important because most of the biological tissue possesses their molecular fingerprints within this spectral range. Therefore, this region of electromagnetic spectrum is extremely useful to determine a tissue spectral map which provides very important information concerning the existence of the critical diseases such as cancer. The W-type chalcogenide fiber structure reported in this paper is a promising candidate for the development of the coherent broadband mid-IR supercontinuum sources which have potential applications in early cancer diagnostic, food quality control, gas sensing, and imaging.
Chromatic dispersion controlling is crucial in designing practical optical communication systems, and nonlinear systems. Ultra-flattened chromatic dispersion has been numerically performed in silica photonic crystal fibers (PCFs) whose chromatic dispersion variation can be as small as ±0.5 ps/km/nm. However, keeping arrays of air holes at precise sizes and shapes is highly required to realize the targeted dispersion. Consequently, this requires much effort in controlling air pressure during fiber fabrication and is considered as a disadvantage of PCFs. In this report, we propose a novel fiber structure for flexible controlling of chromatic dispersion. The fiber structure is obtained by adding six solid rods around the core of a step-index fiber. Ultra-flattened and close-to-zero chromatic dispersion can be realized by using this fiber structure. The variation of chromatic dispersion from 2.5 to 3.7 μm is as small as 0 ± 0.2 ps/km/nm. Using a laser pumping at 2 μm into a 5-cm-long fiber, highly coherent supercontinuum (1.2 – 3.3 μm at -20 dB level) is experimentally generated.
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